Electrochemical cells, such as lithium batteries, may contain a non-aqueous electrolyte that includes an organic solvent. Typically, a supporting electrolyte, such as lithium hexafluorophosphate (LiPF6) is dissolved in an organic solvent, e.g., a high dielectric constant cyclic carbonate compound such as ethylene carbonate or propylene carbonate. Also, various volume ratios of low viscosity ester solvents such as diethyl carbonate and dimethyl carbonate may be used to increase ion mobility and provide for a higher conductivity solvent mixture.
Prior art electrochemical cells with similar mixed solvent electrolytes have focused primarily on increasing lithium ion conductivity through the use of combinations of high dielectric constant solvents and low viscosity co-solvents. However, these prior art electrochemical cells suffer from degraded cell performance caused by the anode reducing the solvent and producing an insulating film from these reduction products. Whenever such an insulating film occurs, the anode becomes electrochemically insulated and prevents lithium ions from migrating to the anode when a load is applied to the cell. Similar problems also occur when the solvent oxidizes and decomposes into products that deposit onto the cathode and also produce insulating layer that negates cell operation. Prior art lithium cells lack the ability to prevent formation of the insulating film and resulting electrode impedance.
One particular example of an organic solvent mixture for a battery electrolyte that sought to reduce internal resistance is U.S. Pat. No. 6,566,015. That patent describes a composition that includes at least one type of a cyclic carbonate compound, an alkyl mono-carbonate compound, an alkylene bis-carbonate compound, a glycol diether compound and a phosphorous-containing organic compound. The use of the gycol diether compound is said to lower the internal resistance of the battery as a result of increasing the mobility of lithium ions at the solid-liquid interface. However, it has been found that this composition suffers from the drawback that it is rather complex and difficult to produce.
Thus, there has been a long-felt need for a relatively simple organic solvent mixture for lithium batteries that provides for increased electrochemical cell performance properties, including prevention of the deleterious insulating film with and an increased energy density output of the cell that does not suffer from the disadvantages, limitations and shortcomings of prior art lithium batteries.